The present invention relates to a binder composition for positive electrode, a slurry for positive electrode using the binder composition, and to a positive electrode and a lithium ion secondary battery using the slurry for positive electrode.
In recent years, secondary batteries have been used as power sources for electronic devices such as laptop computers and cellular phones, and development of hybrid vehicles and electric vehicles using a secondary battery as a power source for the purpose of reducing the environmental load has been promoted. A secondary battery having high energy density, high voltage and high durability is required for these power sources. Lithium-ion secondary battery is attracting attention as a secondary battery capable of achieving high voltage and high energy density.
Lithium ion secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the positive electrode includes a positive electrode active material, a conductive assistant, a metal foil, and a binder. As the binder, a fluorine-based resin such as polyvinylidene fluoride and polytetrafluoroethylene, a styrene-butadiene copolymer, and an acrylic copolymer are used (for example, refer to Patent Documents 1 to 3).
However, in general, there had been cases where the conventional binder showed poor binding property with the metal foil. In addition, in order to increase the voltage and the energy of the lithium ion secondary battery, a positive electrode active material having a high potential is required. Usually, the conventional binder have poor oxidation resistance, and thus when charge and discharge were repeated, the binder was decomposed and the positive electrode active material would fall off from the metal foil, thereby resulting in decrease in the battery capacity.
In order to solve these problems, Patent Document 4 discloses a binder composition for positive electrode in which polyacrylonitrile is graft-copolymerized with polyvinyl alcohol, and when this positive electrode binder is used, lithium ion secondary showed improved durability and oxidation resistance. However, in the current market, a lithium ion secondary battery with even higher durability is desired when used in combination with a high potential positive electrode active material, and thus a positive electrode binder that satisfies this requirement is necessary.